Added an import onnx statement to __init__.py to fix MacOS bug

Author: TomWildenhain-Microsoft

tf2onnx/__init__.py

@@ -9,6 +9,7 @@
 __all__ = ["utils", "graph_matcher", "graph", "graph_builder",
            "tfonnx", "shape_inference", "schemas", "tf_utils", "tf_loader"]
 
+import onnx
 from .version import version as __version__
 from . import verbose_logging as logging
 from tf2onnx import tfonnx, utils, graph, graph_builder, graph_matcher, shape_inference, schemas  # pylint: disable=wrong-import-order